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================================================================================================ Frequency dispersion in groups of gravity waves on the surface of deep water. The red dot moves with the phase velocity, and the green dots propagate with the group velocity. In this deep-water case, the phase velocity is twice the group velocity. The red dot overtakes two green dots when moving from the left to the right of the figure. ================================================================================================ The velocity of a wave can be defined in many different ways, partly because there many different kinds of waves, and partly because we can focus on different aspects or components of any given wave. The ambiguity in

>> What is exactly PCM? Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems. A PCM stream is a digital representation of an analog signal, in which the magnitude of the analogue signal is sampled regularly at uniform intervals, with each sample being quantized to the nearest value within a range of digital steps. For digital communication, there are two quite different PCM standards. The North American standard,

>> Nonlinear Effects in High Power, High Bit Rate Fiber Optic Communication Systems When optical communication systems are operated at moderate power (a few milliwatts) and at bit rates up to about 2.5 Gb/s, they can be assumed as linear systems. However, at higher bit rates such as 10 Gb/s and above and/or at higher transmitted powers, it is important to consider the effect of nonlinearities. In case of WDM systems, nonlinear effects can become important even at moderate powers and bit rates. There are two categories of nonlinear effects. The first category happens because of the interaction of light

>> What is Jitter and Wander? 1. Jitter: Jitter is the short-term phase variations of the significant instants of a digital signal from their ideal positions in time. It is the deviation of the significant instants of a digital signal from the ideal, equidistant values. The significant instant can be any convenient, easily identifiable point on the signal such as the rising or falling edge of a pulse. Otherwise stated, the transitions of a digital signal invariably occur either too early or too late when compared to a perfect square wave. 2. Wander: A second parameter closely related to jitter

Most semiconductor lasers have cleaved facets that form the optical cavity. The facets are perpendicular to the surface of the wafer and light is emitted parallel to the surface of the wafer. But for many applications requiring a two-dimensional laser array or monolithic integration of lasers with electronic components, e.g., optical interconnects, it is desirable to have the laser output normal to the surface of the wafer. Such lasers are known as surface emitting lasers (SEL). A class of surface emitting lasers also has an optical cavity normal to the surface of the wafer. These devices are known as vertical-cavity